Whenever someone finishes taking a shit and decides to lazily leave their toilet paper in the trash can, but in this case the person leaves the toilet paper with the shitty side facing upwards for all to bask in its glory.

Sunnyside up is intentionally NOT flushing the toilet and leaving a massive turd and toilet paper with the poop smear side facing up.This is done to irritate your significant other or to achieve that trendy "gas station restroom" ambience for your own bathroom.

Wife: "You nasty,gross bastard! You didn't flush the toilet and you left the toilet paper sunnyside up."